One-shot circuit with short retrigger time



July 1, 1969 R. s. HUGHES 3,453,453

ONESHOT CIRCUIT WITH SHORT RETRIGGER TIME Filed July 12, .1966 Sheet of 2 acuum. cc

24 v (V Icom'RoL VOLTAGE gulf-lg @0475! g OUTPUT W 2-9|5 2N9l6 Q .OOI/lf mans mPuTo mnsssn IOK g5 I INVENTOR. FIG 2 ll? RICHARD s. HUGHES ROY MILLER ATTOR NEY July 1,1969 I Filed July cmoo,

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R. s. HUGHES 3,453,453

ONE-SHOT CIRCUIT WITH SHORT RETRIGGER TIME Sheet I? of 2 200 CALCULATED VALUES MEASURED VALUES FIG. 3.

I so coumon. VOLTAGE v FIG. .4.

MEASURED VALUES CALCULATED VALUES 1 I 1 1 l 1 L5 2 2.5 3 4 CONTROL VOLTAGE v INVENTOR,

RICHARD s. HUGHES ROY MILLER ATTORNEY United States Patcnt US. Cl. 307-273 4 Claims The invention described herein may -be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a monostable multivibrator circuit having a very short retrigger time.

The conventional monostable multivibrator circuit configuration is well-known in the art of triggered circuits and is frequently referred to as a one-shot circuit. In triggered circuits of the one-shot type, the quiescent point of the monostable mode of operation is in one of the stable regions. When the one-shot circuit is triggered by an external pulse, its operating point is moved from the normal stable region to the temporary stable region. The time constant of the circuit elements holds the operating point in the temporary stable region for a finite period of time. The operating point then moves back to the original, normal stable region.

For many pulse and switching circuit applications, the trigger time of the one-shot circuit should be very short The trigger voltage necessary to turn on the one-shot circuit should not be a function of the period of the trigger pulses. Also, the one-shot circuit should have a variable period that is an accurate, simple function of an external direct current control voltage. Furthermore, the periodcontrolling elements of the circuit preferably should draw current only during the recovery time of the circuit to thus keep the current drain from the voltage supply to a minimum.

Accordingly, it is an important object of this invention to provide a one-shot circuit having a very short retrigger time.

Another object of this invention is to provide a oneshot circuit wherein the trigger voltage required to turn on the circuit is not a function of the period of the trigger pulses.

A further object of this invention is to provide a oneshot circuit having a variable period which is a hyperbolic function of an external, direct current control voltage.

Still another object of this invention is to provide a variable period one-shot circuit wherein the period-controlling elements of the circuit draw current only during the recovery time of the circuit and thus keep the current requirements of the voltage supply to a minimum.

Additional objects of the invention will become apparent from the following description which is given primarily for purposes of illustration, and not limitation.

Stated in general terms, the objects of the invention are attained by providing a one-shot circuit which includes a conventional monostable multivibrator circuit configuration and three external transistors. A first external transistor supplies base-bias current to the normal state saturated transistor of the conventional monostable multivibrator circuit configuration and serves as a con stant current source for the capacitor of the said multivibrator circuit configuration. A second external transistor is coupled to one of the terminals of the capacitor in the said multivibrator configuration and to a supply voltage to charge the capacitor in the normal state and a third external transistor is coupled to a control voltage and to the other terminal of said capacitor for charging the 3,453,453 Patented July 1, 1969 capacitor after it has been discharged by trigger action. A diode preferably is coupled to the transistor that is normally off in the said conventional multivibrator configuration to isolate the capacitor charging path from the R resistor in the off state of said transistor.

A more detailed description of a specific embodiment of the invention is given below with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic circuit diagram showing a oneshot circuit in accordance with the invention;

FIG. 2 is a similar diagram showing an actual circuit of the invention that was constructed and operated;

' FIG. 3 is a graph showing the period of the circuit of FIG. 2 as a function of the external control voltage of the one-shot circuit when the capacitor thereof has a capacitance value of 0.001 microfarad; and

FIG. 4 is a similar graph obtained when the capacitor has a capacitance value of 0.033 microfarad.

In the one-shot circuits shown in FIGS. 1 and 2, the retrigger time is much shorter than that of conventional one-shot circuits, being about one-twentieth the retrigger time of such conventional circuits. The period of the circuits of FIGS. 1 and 2, respectively, is a hyperbolic function of the control voltage in each case. In addition, the period-controlling elements of these circuits draw current only during the recovery times of the circuits, thus keeping the current drain from the voltage supply source down to a minimum value. Furthermore, in these circuits the trigger voltage necessary to turn the circuits on is not a function of the period.

The variable period monostable multivibrator of FIG. 1 has a basic configuration which differs from that of the conventional one-shot circuit in containing a diode D connected in series with the customary capacitor C Diode D isolate the capacitor C charging path from resistor R when transistor Q is turned off, thus enabling voltage V to have a fast fall time, or trailing edge. In addition to capacitor C the conventional monostable multivibrator circuit configuration contains transistors Q and Q conventionally interconnected.

In the normal state, in the absence of a trigger pulse, transistor Q is saturated and transistor Q, is turned off. The base bias current of transistor Q is supplied from transistor Q which also serves as a constant circuit source (for capacitor C The low collector voltage on transistor Q, is fed back to the base of transistor Q thus keeping transistor Q turned off.

Transistor Q also is turned oif in the normal no trigger state. Diode D connected to the base element of transistor Q serves to prevent voltage at the base-emitter, V Q, from becoming excessively back-biased. In this normal state, transistor Q charges the capacitor C to the voltage V -V Q When the capacitor C is charged to this voltage, transistor Q turns off. Thus, after capacitor C is charged, it draws no current, and the period and fast-recovery circuit (Q and Q only draw current from the voltage supply during the charge time of capacitor C The only transistors that conduct during the normal, off state are transistors Q and Q When a positive trigger pulse is applied to the base of transistor Q in the one-shot circuit of FIG. 1, through coupling capacitor C and diode D transistor Q turns on. Capacitor C discharges through transistor 0,, and resistor R the voltage V drops to ground, thus pulling voltage V Q at the base of transistor Q down to V -V Q and turning off transistor Q When the voltage V Q at the base of transistor Q is greater than the voltage V -V Q transistor Q turns ofi. With voltage V low, the constant current discharge for capacitor C is through transistor Q The period for the circuit of FIG. 1 can be approxi- FIG. 2 illustrates an actual circuit of the invention. It specifies the circuit components employed, the waveforms at various places in the circuit and other operating conditions. The Zener diode 1N75'5 serves to ensure the transistor Q turns off when transistor Q turns off. The diode connected to the base element of transistor Q serves to prevent the emitted-base junction of the transistor from breaking down. All resistors are specified in ohms and all measurements made with the circuit of FIG. 2 were made with V the control voltage set at volts. FIGURE 3 graphically presents both the measured values and the calculated values of the period of the circuit of FIG. 2 (C =0.O01 microfarad) as a function of the control voltage and FIG. 3 gives similar results when C was set at 0.033 microfarad.

Obviously many other modifications and variations of the one-shot circuit of the present invention are possible in the light of the teachings given hereinabove. It is therefore to be understood that, within the scope of the appended claims, the invention can be practiced otherwise than as specifically described and illustrated herein.

What is claimed is:

1. A one-shot circuit which comprises:

(a) a conventional monostable multivibrator circuit configuration;

(b) a first external transistor coupled to a supply valtage and to the transistor of said conventional monostable multivibrator circuit configuration, that is saturated in the normal state, for supplying basebias current to said saturated transistor in the normal state, and to serve as a constant current source for the capacitor of said conventional monostable multivibrator circuit configuration;

(c) a second external transistor coupled to the supply voltage and to one ofthe terminals of the capacitor of said conventional rhonostable multivibrator circuit configuration for charging said capacitor in the normal state; and (d) a third external transistor coupled to a control voltage and to the other terminal of the capacitor of said conventional monostable multivibrator circuit configuration for charging said capacitor after it has been discharged by trigger action 2. A one-shot circuit according to claim 1, wherein a diode is connected in the said conventional monostable multivibrator circuit configuration, between said capacitor and the transistor that is turned off in the normal state,

for isolating the charging path of said capacitor from the load resistor of said turned off transistor so the voltage of said capacitor will have a fast fall time.

3. A one-shot circuit according to claim 2, wherein a diode is connected to the base of said second external transistor to prevent voltage at the base-emitter junction of said second external transistor from becoming excessively back-biased. 4. A one-shot circuit according to claim 3, wherein a Zener diode is coupled between the supply voltage and said first external transistor to ensure that said first external transistor turns off when the transistor, that is saturated in the normal state in said conventional monostable multivibrator circuit, turns off.

References Cited UNITED STATES PATENTS 3/1961 Geckle 307-273 8/1965 Norwalt 307-273 US. Cl. X.R. 307270, 313 

1. A ONE-SHOT CIRCUIT WHICH COMPRISES: (A) A CONVENTIONAL MONOSTABLE MULTIVIBRATOR CIRCUIT CONFIGURATION; (B) A FIRST EXTERNAL TRANSISTOR COUPLED TO A SUPPLY VOLTAGE AND TO THE TRANSISTOR OF SAID CONVENTIONAL MONOSTABLE MULTIVIBRATOR CIRCUIT CONFIGURATION, THAT IS SATURATED IN THE NORMAL STATE, FOR SUPPLYING BASEBIAS CURRENT TO SAID SATURATED TRANSISTOR IN THE NORMAL STATE, AND TO SERVE AS A CONSTANT CURRENT SOURCE FOR THE CAPACITOR OF SAID CONVENTIONAL MONOSTABLE MULTIVIBRATOR CIRCUIT CONFIGURATION; (C) A SECOND EXTERNAL TRANSISTOR COUPLED TO THE SUPPLY VOLTAGE AND TO ONE OF THE TERMINALS OF THE CAPACITOR OF SAID CONVENTIONAL MONOSTABLE MULTIVIBRATOR CIRCUIT CONFIGURATION FOR CHARGING SAID CAPACITOR IN THE NORMAL STATE; AND (D) A THIRD EXTERNAL TRANSISTOR COUPLED TO A CONTROL VOLTAGE AND TO THE OTHER TERMINAL OF THE CAPACITOR OF SAID CONVENTIONAL MONOSTABLE MULTIVIBRATOR CIRCUIT CONFIGURATION FOR CHARGING SAID CAPACITOR AFTER IT HAS BEEN DISCHARGED BY TRIGGER ACTION. 